Authors: Yingjie Zhu (University of Michigan), Enrico Landi (University of Michigan)

Active regions are the brightest and most dynamic structures in the solar corona. The nature of the heating mechanisms of active regions, with the complicated magnetic field configurations, remains one of the central problems in the coronal heating mystery. In this study, we investigate the heating of the active region plasma in two different events — a large-scale heating caused by an M-class flare and a small-scale brightening of a few individual coronal loops followed by coronal rains — using spectroscopic observations from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER). During the M-flare decay phase we estimated the proton and other heavy ion temperatures in the post-flare loops and found a consistency with the electron temperature; furthermore, no discernible dependence of the ion temperatures on their charge-to-mass ratios was identified in the post-flare loops. During the individual loop heating, we found denser and cold coronal rains plasma (~0.2 MK), alongside hot and tenuous loop (> 1MK) plasma 15 minutes after the onset of the loop brightening, revealing densities different by one order of magnitude. Ion temperatures and nonthermal velocities were measured in both cold and hot plasma. Additionally, we found evidence of non-equilibrium ionization due to the catastrophic cooling of coronal rain plasma. Our study revealed the different responses of the active region plasma through different heating events.